1. Field of the Invention
The present invention relates to an epitaxial wafer for a GaP, GaAs, or AlGaInP light-emitting diode and to a light-emitting diode fabricated using said epitaxial wafer.
2. Description of the Prior Art
The low power consumption and long life of light-emitting diodes (LEDs) that emit visible light have brought them into widespread use as a light source in various display devices and the like. In recent years technical innovation that has increased the brightness of LEDs and the introduction of new crystal materials that enable the emission of diverse colors have led to a further expansion in the range of applications and the numbers of LEDs in use. Among such LEDs, those using a GaP single-crystal substrate include GaP, GaAs and AlGaInP LEDs.
There are three types of GaP LEDs, categorized according to whether they emit red light, yellow green light, or green light. Each type is fabricated by the liquid phase epitaxial growth method. Since GaP is an indirect transition type semiconductor, in the case of red light or yellow green light LEDs, high brightness is obtained by the introduction of an emission center.
In structure, red light LEDs are comprised of an n-type GaP single-crystal substrate on which are grown an n-type GaP epitaxial layer and a p-type GaP epitaxial layer doped with zinc and oxygen. The Zn--O pair in the p-type GaP epitaxial layer forms the emission center, emitting red light having a wavelength in the order of 700 nm.
A yellow green LED comprises an n-type GaP single-crystal substrate on which, if required, first an n-type GaP buffer layer is formed, followed by the formation of an n-type GaP epitaxial layer, and, as the emission center, a nitrogen doped n-type GaP epitaxial layer, and a p-type GaP epitaxial layer, to form an LED that emits yellow green light having a wavelength in the order of 570 nm.
A green LED comprises an n-type GaP single-crystal substrate on which, if required, first an n-type GaP buffer layer is formed, followed by the formation of an n-type GaP epitaxial layer not doped with nitrogen, and a p-type GaP epitaxial layer. Since the LED therefore has no emission center, it is not as bright as the yellow green LED described above. The LED emits green light having a wavelength in the order of 555 nm.
GaAsP LEDs are fabricated by the vapor phase epitaxial growth method. GaP and GaAsP have different lattice constants, so first vapor phase epitaxy is used to grow an n-type GaAsP compositional gradient layer on the n-type GaP single-crystal substrate, and this is followed by the formation of an n-type GaAsP layer having a constant composition, grown by the same vapor phase epitaxy. A p-n junction is then formed by the diffusion of zinc on the surface of the n-type GasP constant composition layer. By adjusting the As and P component contents in the n-type GaAsP constant composition layer, emission ranging in color from red (in the order of 660 nm) through orange (in the order of 610 nm) to yellow (in the order of 590 nm) may be obtained. With a high P content the semiconductor becomes a indirect transition type, in which case doping with nitrogen as the emission center is used to achieve an increase in emission brightness.
AlGaInP LEDs are usually formed by the metal organic chemical vapor deposition (MOCVD) method. In most cases a GaAs single-crystal substrate is used, but it is also possible to use a GaP single-crystal substrate. Because GaAs has a smaller bandgap than the AlGaInP comprising the active layer, it absorbs emitted light. However, GaP has a larger bandgap than AlGaInP, and therefore does not absorb the emitted light. It is therefore more advantageous to use a GaP substrate, since the fact that the portion of the light emitted by the active layer that is emitted on the substrate side is not absorbed means that brighter emission is obtained.
When a GaP substrate is used, since GaP and AlGaInP have different lattice constants, the MOCVD method is used to form, first, an InGaP, AlGaInP or AlInP compositional gradient layer, then an n-type AlGaInP cladding layer, an AlGaInP active layer, a p-type AlGaInP cladding layer and, if required, a contact layer or window layer. AlInP may be used for the cladding layer. The crystal of contact or window layer may be formed of GaAs, GaAlAs, GaP or the like. Moreover, vapor phase epitaxy or another such method may be used instead of MOCVD to form these layers. By adjusting the composition of the crystal components of the AlGaInP active layer, emission ranging in color from red, in the order of 640 nm, through orange and yellow to green, in the range of 550 nm, may be obtained.
The above GaP, GaAsP and AlGaInP LEDs are each fabricated using an n-type GaP single-crystal substrate doped with Te, Si or the like. However, even when the same process is used to form n-type and p-type epitaxial layers on substrates having the same dopants and the same carrier concentration, there will sometimes be variations in the brightness of the LEDs obtained. As far as is known, the cause was not elucidated.
Although the brightness of such LEDs has been improved through recent improvements in technology, the market is demanding still higher brightnesses. At the same time, with the increasing use of large numbers of LEDs to form large displays, there is also an increasing demand for lower variation in brightness.
The object of the present invention is to provide an epitaxial wafer for a light-emitting diode and a light-emitting diode using the wafer having high brightness and low variation in brightness.